1. Field of the Invention
The present invention relates to an obstacle sensing apparatus, and in particular to an apparatus using an FM-CW wave for sensing whether or not an object reflecting the FM-CW wave is an obstacle by measuring a distance and a relative car speed with respect to a car in front.
2. The Related Art
FIG. 1 schematically shows a well-known obstacle sensing apparatus using an FM-CW wave, and FIG. 2 shows in plan a relationship between a car mounting the obstacle sensing apparatus and a front car.
In FIGS. 1 and 2, the FM-CW wave emitted or transmitted from a radio (electromagnetic) wave transmitting device 21 is reflected from the front car 20 and is received at a radio wave receiving device 22. Those devices 22 and 21 are incorporated in a radar sensor.
The transmitted wave and the received wave are mixed in a mixer (not shown) to produce a beat frequency therebetween which is used for measurement of a distance between cars and a relative car speed with respect to the front car 20. It is to be noted that "car speed" will be hereinafter simply referred to as --speed--.
FIG. 3A and 3B show a waveform of the mixed wave, in which a solid line in FIG. 3A shows the transmitted wave which is a frequency-modulated radio wave in the form of a triangle.
The transmitted wave is reflected by a radio wave reflecting object such as a car and turns into a received wave changed in frequency as shown by dotted lines due to Doppler effect if the object is moving.
By mixing those waves, a waveform of a beat frequency signal as shown in FIG. 3B can be obtained. The beat frequency f.sub.b is given by the sum or difference of a distance frequency f.sub.r and a speed frequency f.sub.b respectively corresponding to a distance and a relative speed with respect to the radio reflecting object according to the following principle equation: EQU f.sub.b =(4 .DELTA.f.multidot.f.sub.m /C) R.+-.(2 f.sub.o /C) V (1)
where
V: Relative speed (m/sec) of the reflecting object to the radar sensor;
R: Distance (m) to the reflecting object from the radar sensor;
C: Light velocity (m/sec);
.DELTA.f; Frequency modulation width (Hz);
f.sub.m ; Modulation frequency (Hz);
f.sub.o ; Radar carrier frequency (Hz).
Now assuming that the beat frequency of the increased modulation frequency is f.sub.up and the beat frequency of the decreased modulation frequency be f.sub.dn, the following equation is given: EQU f.sub.r =0.5 (f.sub.up +f.sub.dn) EQU f.sub.d =0.5 (f.sub.up -f.sub.dn) (2)
From Equations (1) and (2), the following equation is obtained: EQU V=(C/2 f.sub.o) f.sub.d EQU R=(C/4 .DELTA.f.multidot.f.sub.m) f.sub.r ( 3)
From Equation (3), the distance R and the relative speed V with respect to the reflecting object which is a front car can be determined.
This FM-CW type obstacle sensing apparatus carries out frequency analysis for the beat signal of the transmitted wave and the received wave. The frequency analysis means, as shown in FIG. 4, that the increased portion and the decreased portion of the beat frequency in the frequency modulated wave are respectively subject to FFT (Fast Fourier Transform) to determine a frequency peak position corresponding to a relative speed and a distance with respect to a radio wave reflecting object such as a car. The combination of the peaks of the increased and decreased frequency portions of the modulated wave is then found out to calculate the distance from and the relative speed of the radio reflecting object from the above Equations (1)-(3).
In the nature of a radio wave, however, if there is a radio wave reflecting object other than a car, the reflected wave from the object cannot be distinguished from a reflected wave from a car whereby the object must be judged as a car, resulting in erroneous information or an erroneous alarm for a driver.
For the solution of this problem, as shown in FIG. 5A, there has been proposed a prior art apparatus in which a radio wave reflecting off an object such as a guardrail or a sound barrier other than a car is recognized upon frequency analysis by detecting frequency peak configurations and particularly by detecting that a frequency band width of the frequency peak is narrower for the car.
However, this prior art apparatus has a radio wave measuring portion which is affected by system noise. Particularly, when frequency peaks of such system noise are superposed with frequency peaks for a car, the band width of the frequency peak of the car is substantially widened, so that whether or not the object is a car can not be decided.
Namely, referring to FIG. 5A, if a frequency peak with respect to a reflecting object such as a car, a frequency peak with respect to a stationary object such as a guardrail or a sound barrier, and system noise peaks are separated from each other in terms of frequency, it will be possible to distinguish a car from a stationary object on the basis of the band widths of the peak frequencies.
However, as shown by dotted lines in FIG. 5B, when the level of system noise is increased due to thermal characteristics of the system, the frequency band is correspondingly widened while at the same time the peak frequency of a car is shifted in response to the movement of a car, the noise peak and the peak of a car can be superposed in terms of frequency. Therefore, the band width of the superposed peak is widened, which prevents distinguishing between a car and a guardrail.